1. Field of the Invention
The present invention relates to a device for adjusting an air gap between lenses, and, more particularly, to a device for adjusting the air gap between lenses to improve the lens performance and the focusing characteristic without dismantling the lenses from a lens barrel.
2. Description of the Related Art
The lens system of a conventional camera is constructed by combining several lenses made of various optical glasses having different dispersion coefficients and refractive powers. When the lenses are correctly arranged within the lens barrel, the lens system can produce a clear, regular, sharp and bright image. Therefore the air gap i.e., the interval between the lenses is an important parameter which influences the performance of the lens system, including changing the position of the image plane within the optical system, etc.
The second lens group of a zoom lens system having two lens groups or the third group of a zoom lens system having three lens groups is normally referred to as a compensator. A change in the air gap within the compensator, for example, will influence the back focus distance at a telephoto position and, therefore, deteriorate the performance of the zoom lens system. For example, in a zoom lens for a lens shutter camera having a high magnification, such a change in the air gap within the compensator will affect the back focus distance. That is to say, a change in the air gap within the compensator will change the back focus distance fifteen times at the telephoto position in a zoom lens having two times magnification and about thirty times in a zoom lens having three times magnification. If the normal tolerance of the air gap within the compensator is .+-.0.05 mm, the tolerance of the back focus distance becomes .+-.0.75 mm for the zoom lens having two times magnification and .+-.1.50 mm for the zoom lens having three times magnification.
Under these circumstances, investigations are performed on how to minimize the tolerance of the air gap between the lenses. Known methods for maintaining the air gap between lenses include: making peripheral edges of the lenses contact each other outside of the effective diameter of the lenses; making centering surfaces of concave lenses contact each other in face-to-face contact; placing a space adjusting member such as a spacer or ring between the lenses; and forming the shape of the lens barrel properly.
FIG. 5 shows a conventional device for adjusting the air gap having a spacer disposed between lenses. Three lenses 52, 54 and 56 are arranged within a lens barrel 50 and fastened by means of a retainer 58. The air gap between the lenses 52 and 54 is maintained by inserting an annular spacer 60 of a predetermined thickness between them. Examples of normally used spacers 60 are shown in FIGS. 6 and 7. Because it is difficult to compensate for the tolerance of the air gap between the lenses during the process of assembling the lens barrel, the spacing may not be exactly correct. If the spacing error is too great to be overcome by a focusing process, the lenses must be dismantled from and assembled back to the lens barrel in order to replace the spacer 60 with another spacer of different thickness. This inconvenient procedure results in a low productivity. And also, because only spacers of definite thickness are available, it is not always possible to adjust the air gap to an optimum value. Thus, the change of the back focus distance may go beyond the tolerance limit, deteriorating lens performance.